As a foundry mold for producing a casting, known are a self-hardening foundry mold, a heat-hardening foundry mold, a gas-hardening foundry mold, and others. The self-hardening foundry mold, which has been most frequently used, is obtained by: adding, to refractory particles such as silica sand, a binder composition for making self-hardening foundry molds containing a furan resin and others, and a hardener including phosphoric acid, an organic sulfonic acid, sulfuric acid or some other acid; mixing these components; and then filling the resultant mixed sand composition into an original pattern such as a wooden pattern.
As the furan resin, for example, the following is used: furfuryl alcohol, condensate made from furfuryl alcohol, furfuryl alcohol/urea formaldehyde resin, furfuryl alcohol/formaldehyde resin, furfuryl alcohol/phenol/formaldehyde resin, or some other known modified furan resin.
WO 90/02007 and JP-A-55-120445 describe, as an additive for a binder composition, an ion compound such as a sulfite salt or a thiosulfate salt.
WO 90/02007 describes that a foundry mold material containing a refractory aggregate and a hardening binder, as main constituent components, in which the hardening binder contains a polyfunctional acrylamide having two or more ethylenically unsaturated groups in the molecule is excellent in low-temperature rapid hardenability, disintegratability and bench life, and is suitable, in particular, as a material for foundry molds for casting a low-melting-point metal such as an aluminum alloy. WO 90/02007 describes that the hardening binder is a polymerizable organic compound having a hardening mechanism different from that of conventional binders, and as a redox catalyst, sulfite salts such as sodium hydrogensulfite, metal soaps such as cobalt naphthenate, and others are used.
JP-A-55-120445 describes a composition for foundry molds that is an acid-hardening resin composition comprising an organic resin hardenable with an acid at normal temperature and a latent hardening agent for the organic resin, in which the latent hardening agent is at least one selected from the group consisting of sulfur compounds which can each form into sulfur trioxide by effect of ozone. JP-A-55-120445 describes, as the latent hardening agent, sulfite salts, hydrogensulfite salts, pyrosulfite salts, hyposulfite salts, aldehyde adducts of hyposulfite salts, thiosulfate salts, and others.